Bethke, in his book 'Game Development and Production', chapter 8, lists the following as game assets: 2D sprites, 3D models, missions, levels, areas, voice, key framing and motion capture, sound effects, music and special effects.
Wikipedia lists the following definition: 'Game assets are the "things" that go into a game. Some examples of assets are artwork (including textures and 3D models), sound effects and music, text, dialogue and anything else that is presented to the user.'
Premnath, J. M., writing for the Deccan Herald states that "the concept artist sketches the concepts for the levels in the game, characters, vehicles and other visual element [sic] such as props... [which] once finalised... materialise into game assets, which can be plugged into the game engine." In summary, he writes that "game assets are everything that contributes to the visual appearance of the game."
Ben Carter, in his book 'The Game Asset Pipeline', (p. 2) describes game assets as "artwork, sounds, video, maps, and other data."
Aside from Premnath, who restricts his definition to include solely the visual elements, game assets are described as being any piece of data which:
a) is in a format that can be used by the game engine,
b) will be presented to the user.
This constitutes everything which could be included in the final game aside from code, scripts and documentation.